A technology has been proposed for a pedestrian protection. It is determined whether or not a collision object with which a vehicle collided is a pedestrian, and an apparatus for protecting the pedestrian is activated when a collision victim is determined to be a pedestrian. In addition, a study to put the pedestrian protection technology to practical use is also being conducted. For this reason, another technology is also desired as a technology to determine whether or not a body to which a vehicle is approaching is a pedestrian. Such a technology is expected to be capable of detecting approach made by a vehicle to a vehicle made of metals prior to a collision in order to protect passengers of the vehicle.
As a conventional pedestrian recognition technology, an electrostatic capacitance method for detecting a body approached by a vehicle has been proposed in JP-A-2000-177514 (U.S. Pat. No. 6,561,301) and JP-A-2000-326808. The electrostatic capacitance method for detecting a body approached by a vehicle is a method for detecting approach to a body exhibiting conductivity. This electrostatic capacitance method electrically detects a difference in electrostatic capacitance to discriminate between a pedestrian as a collision object and a conductor also as a collision object based on a change in electrostatic capacitance between the conductor and an electrode plate of a sensor unit.
It is also conventional to radiate ultrasonic waves and receive reflected ultrasonic waves for detecting approach to an object against the vehicle in a non-contact way.
In accordance with the electrostatic capacitance method for detecting a body approached by a vehicle, however, the difference in electrostatic capacitance between a human body and a conductor is small. Further, the gradients of the sensor outputs for a human body and a conductor are similar. Thus, this electrostatic capacitance method cannot differentiate a human body and a conductor from each other for certain shapes and types of the metallic body. In addition, in order to amplify output changes accompanying changes in electrostatic capacitance, an operation at a high frequency is required. In consequence, effects of high-frequency noises superposed on the circuit output and effects of foreign noise electric waves increase, thus lowering precision of discrimination.
Furthermore, the ultrasonic wave method is difficult to provide an ultrasonic wave oscillation receiver around the whole vehicle from the standpoint of costs. Thus, the ultrasonic wave method has been put to practical use only in a limited way as a back sonar or the like. Moreover, the ultrasonic wave method cannot recognize the type of an approaching object.